Conventional narrow chord fan blades for gas turbine engines comprise solid metal.
One conventional wide chord fan blade comprises a concave metal wall portion, a convex metal wall portion and a honeycomb between the two metal wall portions. This wide chord fan blade is produced by hot forming the wall portions into concave and convex shapes respectively, placing the honeycomb between the metal wall portions and brazing, or activated diffusion bonding, the metal wall portions together around the honeycomb. The interior of the fan blade is evacuated.
Another conventional wide chord fan blade comprises a concave metal wall portion, a convex metal wall portion and metal walls extending between the two wall portions. Placing a metal sheet between two tapered metal sheets and diffusion bonding the sheets together at predetermined positions to form an integral structure produces this wide chord fan blade. Then inert gas is supplied into the interior of the integral structure to hot form the integral structure into a die to produce the concave and convex walls and the walls extending between the concave and convex walls. The interior of the fan blade is evacuated.
A disadvantage of a wide chord fan blade is that it is not as stiff as a narrow chord fan blade. The reduced stiffness results in an increased risk of stalled flutter within the operating range of the gas turbine engine and an increased susceptibility to other forms of vibration. A further disadvantage of the wide chord fan blade is that it is very expensive and time consuming to produce.
One solution to damping the vibrations of a fan blade is to fill the interior with a viscoelastic material core, bonded to the interior of the wall portions, as disclosed in the UK Patent Application GB0130606.7. However, the structural requirements of a fan blade and thus the core are to resist rotational, vibrational and impact loads. A disadvantage of this system is that rotational and impact loads require a core of high strength and high stiffness, whereas for vibration damping the core is preferably of a low modulus. A further disadvantage is that the damping ability of the viscoelastic material core is compromised by the necessity to reduce the parasitic weight, which is achieved by inclusion of microbubbles.